1. Field of the Invention
The present invention relates to a liquid crystal display device which is used for a display for personal computer, a television set, a projection projector, and the like, and particularly to a liquid crystal display device which has excellent response characteristics and is preferable in displaying moving images.
2. Description of the Prior Art
Liquid crystal display devices have advantages that they are thin and light, that they can be operative at low voltages, and that they have low power consumption. Accordingly, liquid crystal display devices are widely used in various kinds of electronic devices. In particular, active matrix liquid crystal display devices in which a thin film transistor (TFT) is provided as a switching element for each picture element show excellent display characteristics comparable to those of cathode-ray tube (CRT) displays, and therefore have come to be used not only for displays for personal computers, but also for television sets, projection projectors, and the like.
In general, a liquid crystal display device has a structure in which liquid crystals are sealed between two substrates being disposed to face each other. On one substrate, a TFT, a picture element electrode, and the like are formed, while color filters, a common electrode, and the like are formed on the other substrate. Hereinafter, a substrate on which a TFT, a picture element electrode, and the like are formed is referred to as a TFT substrate; and a substrate, which is disposed to face the TFT substrate, is referred to as an opposing substrate. A structure formed by sealing liquid crystals between the TFT substrate and the opposing substrate is referred to as a liquid crystal panel.
Polarizing plates are disposed respectively on both sides of a liquid crystal panel in a thickness direction thereof. By applying a voltage between a picture element electrode and a common electrode, an alignment state of liquid crystal molecules is changed so that the amount of light passing through these polarizing plates can be adjusted.
Heretofore, twisted nematic (TN) liquid crystal display devices have been widely used in which liquid crystals with positive dielectric anisotropy are sealed between two substrates and in which liquid crystal molecules are twisted and aligned. However, the TN liquid crystal display devices have a disadvantage that viewing angle characteristics are poor and that color contrast and color tone change to a large extent when the screen is viewed from an oblique direction. Accordingly, multi-domain vertical alignment (MVA) liquid crystal display devices, which have favorable viewing angle characteristics, have been developed and put into practical use.
In MVA liquid crystal display devices, liquid crystals with negative dielectric anisotropy are sealed between two substrates, and alignment control structures are disposed in order to form a plurality of domains in which alignment directions of liquid crystal molecules are different from one another in one picture element, when a voltage is applied. For the alignment control structures, for example, protrusions formed of dielectric materials and slits of electrodes are used.
FIG. 1 is a view showing an equivalent circuit for one picture element of a liquid crystal display device. As shown in FIG. 1, one picture element of the liquid crystal display device includes a TFT 10, a liquid crystal cell CLC, and an auxiliary capacitance Cs. The liquid crystal cell CLC includes a picture element electrode, a common electrode, and liquid crystals interposed therebetween.
The TFT 10 is turned on/off by scanning signals supplied to a gate bus line 11. When the TFT 10 is turned on, display signals (display voltages) are supplied from a data bus line 12 to the liquid crystal cell CLC and the auxiliary capacitance Cs. Thereafter, even when the TFT 10 is turned off, the voltages held in the liquid crystal cell CLC and the auxiliary capacitance Cs are still applied to liquid crystals.
In liquid crystal display devices, after the voltage is applied between a picture element electrode and a common electrode, it takes time for all the liquid crystal molecules within a picture element to align in predetermined directions in accordance with the voltage. In addition, since liquid crystal molecules have dielectric anisotropy, a capacitance value of the liquid crystal cell CLC changes until the time at which all the liquid crystal molecules are aligned in predetermined directions after the voltage is applied. Consequently, the voltage applied to the liquid crystals decreases. Therefore, as shown in FIG. 1, the auxiliary capacitance Cs is connected to the liquid crystal cell CLC in parallel thereto so that a change in the voltage applied to the liquid crystals becomes small.
However, conventional liquid crystal display devices have a problem that after-images occur when displaying moving images, since response characteristics are not sufficient. FIG. 2 is a view showing the response characteristics of a conventional liquid crystal display device, with time after a first display signal is applied on the horizontal axis and with transmittance (luminance) on the vertical axis. As shown in FIG. 2, in the conventional liquid crystal display device, when the display is changed from a black display state to a white display state, a desired transmittance is not achieved when the first display signal is applied, and, in many cases, the desired transmittance is achieved when the second display signal is applied. In general, when transmittance in a white display is set to 100%, a response time is defined by time tr (rise time) which is required for the transmittance to change from 10% to 90%, and by time tf (fall time) which is required for the transmittance to change from 90% to 10%.
For improving response characteristics of liquid crystal display devices, an improvement of liquid crystal materials may be conceived. However, any liquid crystal materials, which have satisfactory response characteristics and which satisfy both of display capability and long-term reliability, have not so far been obtained.
It is also conceivable that the capacitance value of the auxiliary capacitance Cs is increased, and, thereby, decreasing an applied voltage due to dielectric anisotropy of liquid crystal molecules can be suppressed. However, in general, since an electrode constituting the auxiliary capacitance Cs is formed of metals, enlarging the electrode to increase the capacitance value results in decreasing an aperture ratio, and, hence, the screen becomes dark.
In coping with the above problems, a technology so-called overdrive, which improves response characteristics by using contrived driving techniques, was developed. This technology is that, for example, in a case of a liquid crystal display device in normally black (NB) mode, when the display is changed from a black display to a halftone display, a state change of liquid crystal molecules is accelerated by changing a voltage in three steps from a black display voltage (low voltage) to a white display voltage (high voltage), and to a halftone voltage (intermediate voltage).
In Japanese Patent Application Laid-open No. 2001-343956, it is descried that, in a liquid crystal display device in normally white (NW) mode, an overdrive driving is performed. In this liquid crystal display device, for example, between a black display (display voltage 5 V) and a white display (display voltage 2.2 V), a voltage (1.9 V) lower than the white display voltage is applied only for a period of one frame.
However, the overdrive has a disadvantage that since it is necessary to change a voltage supplied to data bus lines in three steps from a black display voltage to a white display voltage, and to a halftone voltage, driving circuits become complex. In addition, in an MVA liquid crystal display device in normally black mode, when the display is changed from a black display to a halftone display, a response time can be shortened by an overdrive; and, however, when the display is changed from a black display to a white display, a voltage higher than that in a white display can not be applied so that a response time can not be shortened.
In Japanese Patent Application Laid-open No. 2003-172915, it is described that, when the display is changed from a black display to a white display, a voltage higher than a white display voltage (highest tone voltage) is applied. However, in that case, it is also necessary to change a display voltage in three steps. In addition, it is necessary to form a TFT which has a high withstanding voltage, and thereby, it causes a problem that it is necessary to modify a design and processing.
In Japanese Patent Application Laid-open No. 2000-231091, when a display is changed from a black display to a halftone display, a voltage higher than a targeted voltage of a halftone display is applied. In that case, it also has a disadvantage that since it is necessary to change a voltage supplied to data bus lines in three steps, a driving circuit become complex.